(1) Field of the Invention
The present invention relates to the field of nuclear medicine instrumentation. More specifically, the present invention relates to the field of signal processing within a gamma camera system.
(2) Prior Art
Gamma cameras of the Anger type are well known and commercially available. As is known, these camera systems contain a detector having a plurality of photomultiplier tubes (PMTs) arranged in a matrix within a detector head. Each PMT (e.g., channel) generates an output signal responsive to light energy associated with a scintillation event. This signal along with all PMT signals of the matrix are then typically integrated over time in order for the camera system to obtain an energy reading of the scintillation event. In an amplification circuit stage, each channel of the prior art systems has an associated single integration unit that integrates the PMT output signal. For each scintillation event, all channels of the detector head output a response signal that is integrated over time. For those channels closest to the event, their signals are larger then the other PMT channels. The energy response of a PMT is known and is a decaying exponential with a time constant of T. As is known, after an integration period of 5 time constants, 5 T, substantially all of the usable energy of the scintillation event is dissipated. However, there is some time period less than 5 T over which the energy level a scintillation event may be validly measured to obtain a valid indication of the event.
A problem with prior art integration techniques occurs during what is known as pulse "pile-up" conditions. Pulse pile up is experienced when scintillation events occur so rapidly in time, that their separation time drops beneath the 5 T period. In effect, a second event occurs during the integration period of a first gamma event, while the channels are integrating the first event in order to compute its total energy and spatial coordinate. In the prior art gamma camera systems, when the separation between two gamma events drops beneath a particular value, R, then both events are discarded because each event corrupts the other. Therefore, what is needed is a mechanism to reduce the unwanted effects of pulse pile up at high count rates. The present invention offers such advantageous functionality. What is needed is a system that can provide detection and accurate recording of two gamma events that occur within the time period, R, or that occur substantially simultaneously. The present invention allows such advantageous functionality.
Accordingly, it is an object of the present invention to provide improved recording of energy and spatial location of gamma events at high count rates. It is yet another object of the present invention to reduce the problems associated with gamma cameras during pulse pile up conditions. It is another object of the present invention to provide a mechanism and method for accurately detecting two or more gamma events that occur very closely in time. These and other objects not specifically mentioned above, will become clear within discussions of the present invention to follow.